1. Field of the Invention
The present invention generally relates to wireless communications and, in particular, to a method and apparatus for transmitting/receiving reference signals in Long Term Evolution (LTE) and LTE-Advanced (LTE-A) systems based on the 3rd Generation Partnership Project (3GPP) standards.
2. Description of the Related Art
With the advance of communication technologies, the conventional voice telephony-oriented mobile communication systems have evolved to high speed, high quality packet data-oriented mobile communication systems for providing data and multimedia services. In order to support such high speed, high quality data transmission services, various mobile communication technologies have been standardized. These technologies include 3GPP High Speed Packet Access (HSPA) and LTE, 3GPP2 High Rate Packet Data (HRPD) and Ultra Mobile Broadband (UMB), and IEEE 802.16e.
In order to improve transmission efficiency, these mobile communication technologies use various advanced techniques such as Adaptive Modulation and Coding (AMC) and channel sensitive scheduling. The AMC technique allows the transmitter to adjust the data rate according to the channel condition. The AMC technique decreases the data rate for bad channel conditions so as to maintain a reception error rate at an intended level and increases the data rate for good channel conditions to maximize the throughput. The channel sensitive scheduling technique allows the transmitter to provide the service to a receiver having the best channel condition among a plurality of receivers, thereby improving system throughput.
The AMC and channel sensitive scheduling techniques receive Channel State Information (CSI) fed back from the receivers and perform data transmission at an optimal timing with an optimal modulation and coding scheme. The AMC and channel sensitive scheduling techniques can be referred to as techniques for improving transmission efficiency with the enough information on the transmission channel. In a Frequency Division Duplex (FDD) system in which the transmitter cannot analogize the transmission channel condition from the reception channel, the receiver is designed to feed back transmission channel information to the transmitter. Meanwhile, a Time Division Duplex (TDD) system has a characteristic that the transmission channel condition is analogous to the reception channel condition such that there is no need for the receiver to report the information on the transmission channel to the transmitter.
Recently, research has been conducted to replace the Code Division Multiple Access (CDMA) scheme dominant in the 2nd and 3rd generation mobile communication systems with the Orthogonal Frequency Division Multiple Access (OFDMA) scheme. Actually, the standardization organizations such as 3GPP, 3GPP2, and IEEE are in the middle of standardization of the advanced communication systems adopting OFDMA or modified OFDMA. This is because OFDMA is expected to provide superior throughput as compared to CDMA. One of the main factors that allows OFDMA to increase system throughput is the frequency domain scheduling capability. As channel sensitive scheduling increases the system capacity using the time-varying channel characteristic, OFDM can be used to obtain more capacity gain using the frequency-varying channel characteristic.
The LTE system uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) in the uplink. Both OFDM and SC-FDMA have the characteristics allowing frequency domain scheduling.
In the meantime, the LTE system supports multiple antenna transmission in the downlink (DL). An LTE transmitter can be provided with one, two, or four transmit antennas. By using multiple antennas, beamforming gain and spatial multiplexing gain can be achieved with a precoding scheme.
The LTE-A system can support up to 8 transmit antennas in the downlink. As the number of transmit antennas increases, the transmitter can further improve the beamforming gain and spatial multiplexing gain. In addition, the LTE-A system can support other various transmission techniques in the downlink. One of the newly introduced downlink transmission techniques is Coordinated Multi-Point (CoMP), which improves the communication quality of specific receivers through the cooperation of multiple cells. In terms of downlink CoMP, two different approaches can be considered: Joint Transmission (JT) in which multiple transmission points transmit signals to a single receiver simultaneously and Coordinated Scheduling/Coordinated Beamforming (CS/CB) in which the receiver receives the signal transmitted by its serving cell. However, the scheduling including any beamforming functionality is dynamically coordinated between the cells in order to control and/or reduce the interference between different transmissions. Unlike JT in which multiples cells have to prepare the symbol streams simultaneously, CS/CB can be implemented by exchanging the scheduling and beamforming information among the multiple cells. This means that, although JT is expected to secure higher performance gain, there are many problems to be solved for implementation such as high traffic load and low delay requirement between cells. In contrast, CS/CB gives relatively low performance gain but is advantageous to the low traffic load between cells.